Recently, usage of high strength steel has been continuously increased to reduce the weight of vehicles, but abrasion and fracturing of steel sheets may easily occur if high strength steel is processed at room temperature. In addition, in the middle of processing, a springback phenomenon may occur, whereby it may be difficult to process dimensions precisely. Thus, hot press forming (HPF) is applied as one preferable method of processing high strength steel without defects.
Hot press forming (HPF) is a method of processing a steel sheet at high temperature to have a complex shape by using properties in which the steel sheet is able to be softened and becomes highly ductile at high temperatures and, more particularly, is a method of manufacturing a product having high strength and a precise shape, as a structure of a steel sheet is transformed to a structure of martensite by performing processing and quenching at the same time, after the steel sheet is heated to a temperature beyond that of an austenite region, in other words, in a state in which a phase transition is possible.
Meanwhile, if the high strength steel is heated to a high temperature, a surface defect, such as corrosion, decarburization or the like may occur in a surface of the steel. To prevent the surface defect, after zinc-based or aluminum-based plating is performed on the surface of the steel, hot press forming (HPF) is performed. In this case, zinc (Zn) or aluminum (Al) used for a plating layer serves to protect a steel sheet from the external environment, thereby improving corrosion resistance of the steel sheet.
An aluminum-plated steel sheet has an advantage of not forming a thick oxide film on a plating layer, even at a high temperature, due to a high melting point of Al and a dense and thin Al oxide film formed on an upper part of the plating layer. On the other hand, a zinc-plated steel sheet has an excellent effect of protecting a steel sheet from corrosion, even by a scratch of a cross section or a surface due to self-sacrificing corrosion resistance of zinc. Such self-sacrificing corrosion resistance of the zinc-plated steel sheet is better than that of the aluminum-plated steel sheet. Thus, corrosion resistance improving effects of the zinc-plated steel sheet are better than those of the aluminum-plated steel sheet. Thus, hot press forming (HPF) using the zinc-plated steel sheet on behalf of the aluminum-plated steel sheet, has been proposed.
However, if the zinc-plated steel sheet is heated to a temperature above an austenite transformation temperature to undertake hot press forming, as a heating temperature is higher than a melting point of a zinc layer, in other words, a zinc plating layer, zinc may be in a liquid state for a predetermined time on a surface of a steel sheet. In this case, if such liquid zinc is present on the surface of the steel sheet during processing of the steel sheet in a press, tensile stress may occur in the surface of the steel sheet, whereby a grain boundary of base iron may be drenched with the liquid zinc. The zinc with which the grain boundary is drenched allows binding force of an interface to be weak. Thus, the interface may act as a region in which a crack occurs under tensile stress. A phenomenon in which a propagation velocity of the crack generated in the surface of the steel sheet may be relatively rapid and the crack may be deeply propagated in comparison with base iron according to the related art, may occur.
Such a phenomenon is called known as a liquid brittle fracture, and the phenomenon may cause a problem of material degradation such as a fatigue fracture, bending properties degradation and the like, whereby the liquid brittle fracture should be avoided. To date, in the hot press forming of zinc-plated steel sheets, the problem of the liquid brittle fracture has not yet been fundamentally solved.
Furthermore, to improve corrosion resistance of an aluminum-plated steel sheet or an aluminum-silicon alloy plated steel sheet, a method of alloy plating magnesium (Mg) is used. Since an aluminum-magnesium alloy plated steel sheet and an aluminum-silicon-magnesium alloy plated steel sheet manufactured therefrom have excellent corrosion resistance by itself, such sheets are used for building materials and materials for forming vehicle components.
However, if a plated steel sheet on which Al and Mg are alloy plated is heat treated at a temperature above 900° C. for hot press forming, Mg is diffused toward a surface of a plating layer during the heating process, thereby forming a magnesium oxide (MgO) on the surface. This oxide may have a low degree of adhesion, and a portion of the oxide may be adhered to a forming die, thereby contaminating the die. Furthermore, MgO adhered to a surface of a formed article after forming, may serve as resistance in a process in which the formed article is resistance welded, thereby causing a welding defect.